Estimating latitudinal variability in extreme heat stress on rocky intertidal shores
Aim Broad-scale patterns of heat stress play an important role in shaping the geographical distributions of many species and may differ from large-scale changes in average temperatures. For species living on rocky intertidal shores extreme heat stress occurs when hot dry aerial conditions coincide with midday low tides. We used empirical and modelled temperature data, and estimates of cumulative aerial exposure and solar radiation, in order to test the hypothesis that heat stress on Australian rocky intertidal shores decreases with increasing latitude. Location Rocky intertidal shores of south-eastern Australia spanning > 1500 km and 13° of latitude (26°24′23″ S to 39°07′47″ S). Methods In situ temperature measurements, hourly tidal elevations and daily solar radiation taken over three consecutive summers (December 2009-February 2012) were used to quantify latitudinal variability in extreme heat stress, cumulative aerial exposure and solar radiation, respectively. Comparisons between hourly in situ temperatures and meteorological data were used to produce a large-scale statistical model capable of estimating intertidal substratum temperatures during daytime low-tides, which was then extrapolated across 22 locations. Results Heat stress estimated using in situ loggers deployed across five east coast locations typically did not decline with increasing latitude and neither did midday exposure or solar radiation. The meteorological model proved to be a successful method for estimating rocky shore heat stress and in contrast to the empirical data displayed strong latitudinal trends in mean daily maxima and cumulative heat stress. Modelled acute heat stress (i.e. summer maxima), however, did not decline with increasing latitude, as there was greater thermal variability at higher latitudes. Main conclusions The meteorological model developed in this study represents a useful approach for estimating broad-scale patterns of heat stress on rocky intertidal shores. Results also indicate that latitudinal patterns of acute and chronic heat stress may differ from average temperatures, which are commonly assumed to decline with increasing latitude. Such broad-scale patterns of thermal stress as described in this study will significantly contribute to our ability to understand the impact of climate change on vulnerable rocky intertidal communities.